The present invention relates in general to the control of paper making machines and, more particularly, to modeling and control of sheet weight and moisture for paper machine transitions. While the present invention is generally applicable to control of paper making machines, it will be described herein with reference to control for making grade changes on such machines for which it is particularly applicable and initially being used.
Many paper makers want to make more frequent, faster and smoother grade changes to better adapt their production to market demands. Grade changes typically involve changes of sheet weight, moisture content level, fiber furnish, color, ash content level, and many other paper properties. To change paper properties from one product grade to another usually requires changing chemical additives in the wet-end stock preparation, stock flow, machine speed, headbox settings, steam pressures, and other process variables. Because each of these factors may exhibit different dynamics and have different transport delays during the transition, the machine may take a long time before it settles into a new steady state or the paper sheet may break during the change. The paper produced during a grade change usually does not meet the specifications of either grades of paper and is referred to as un-saleable xe2x80x9cbrokexe2x80x9d. Thus, a smoother grade change, which avoids a sheet break and reduces broke, can definitely increase a machine""s productivity, particularly for a machine that performs frequent grade changes.
An investigation of grade changes on a paper making machine shows that the problems related to grade change are very complex in nature. Some issues of grade changes are related to the characteristics of a paper machine itself. Others are associated with operational techniques and different operators"" approaches. The most common limitations of a paper machine are either machine speed or steam pressures, i.e., the drying capability of the machine, or both. The speed limit or sluggish drying responses may be the main limiting factor for achieving a faster grade change. Occasionally, the wet-end capacity or stock supply can also be the limiting factor. For a machine with a pressurized headbox and Fourdrinier wire, the responsiveness of the headbox and dryline dynamics often are crucial to the performance of a grade change.
Typically, the machine operator""s experience and knowledge play a key role in making a grade change. An operator who is lacking in process knowledge or operational experience tends to make the required changes in an uncoordinated sequence and wait for the resulting responses before performing any further adjustments. Since the process dynamics and transport delay timing can be totally out of synchronization for such a changeover, the process may go through a series of unwanted oscillations. In the worst case, a sheet break could occur and the production would be disrupted. Attempted manual corrective actions can prolong a grade change operation or result in an irregular grade change rather than correct such problems. Even with experienced operators, it is common that each operator will do the same grade change with different settings, different execution sequences, and different adjustments through the transitions. Accordingly, there is a need for a standard operational procedure for a well coordinated grade change, which is consistently used by all operators of a machine. The inventors of the present application have recognized that novel modeling and control of headbox transient responses for sheet weight and moisture can significantly improve on paper machine control and can serve as a base for such a standard operational procedure for grade changes.
The novel modeling and control of headbox transient deviations for sheet weight and moisture of the invention of the present application significantly advance the performance of paper making machines including, for example, during grade changes and speed changes. Applicants have modeled headbox transient responses as a combination of two sets of time constants and dead time delays. One set represents a shorter delay with faster response dynamics, the fast mode moisture and weight transients, and the other models the longer delay with slower dynamics, the slow mode moisture and weight transients. The combination of fast and slow modes forms a basis for controlling weight and moisture transient deviations caused by headbox changes during a paper machine transition. A dynamic and delay time model is determined for operation of a stock valve of the paper making machine and the stock valve is controlled in accordance with the stock valve dynamic model and the transient model of the headbox to compensate for weight and moisture changes which result from headbox changes in a web of paper being manufactured.